THIS invention relates to electronic radio frequency identification (RFID) systems and more particularly to a reader or receiver forming part of such a system.
In known systems of the aforementioned kind, the reader transmits an interrogation signal towards a transponder population. Each transponder comprises an antenna connected to an integrated chip hosting electronic circuitry of the transponder, including a modulator and a controller. In response to the interrogation signal, the transponder modulates, by utilizing backscatter modulation, the impinging interrogation signal with digital data to be transmitted to the reader. This is done by the controller causing the modulator to cause a deliberate mismatch between an input impedance of the chip and an impedance of the antenna, in accordance with the digital data to be transmitted.
The resulting signal is an amplitude modulated signal comprising an upper sideband and a lower sideband on either side of a carrier frequency, which normally is the frequency of the energizing signal.
Most known readers comprise wideband receivers for receiving both sidebands and converting them directly to baseband utilizing the carrier frequency.
A problem with the known systems is that the broadband receiver is susceptible to noise generated by other transponders or another source (e.g. another similar system, a cellular telephone etc) in one or both of the upper sideband and the lower sideband. Even if one sideband is unaffected, the present readers cannot decode the data received.
In U.S. Pat. No. 6,064,320 to d'Hont et al there are disclosed single sideband readers that are used adjacent to each other at a toll plaza for respective lanes of a multi-lane road. The one reader would be a lower sideband reader only and the adjacent reader an upper sideband reader only. The purpose of this alternating arrangement is simply to maintain signal separation between the readers. If the signal in the relevant operative band is bad, the reader cannot decode the data received.